Overview
The present invention is an extension of work assigned to TranS1 Inc. with a principle place of business located in Wilmington, N.C. Much of the work is described in great detail in the many applications referenced above and incorporated by reference into this application. Accordingly, the background of the invention provided here does not repeat all of the detail provided in the earlier applications, but instead highlights how the present invention adds to this body of work.
The spinal column is a complex system of bone segments (vertebral bodies and other bone segments) which are in most cases separated from one another by discs in the intervertebral spaces (sacral vertebrae are an exception). FIG. 1 shows the various segments of a human spinal column as viewed from the side. In the context of the present invention, a “motion segment” comprises adjacent vertebrae, i.e., an inferior and a superior vertebral body, and the intervertebral disc space separating said two vertebral bodies, whether denucleated space or with intact or damaged spinal discs. Each motion segment contributes to the overall flexibility of the spine contributes to the overall ability of the spine to flex to provide support for the movement of the trunk and head.
The vertebrae of the spinal cord are conventionally subdivided into several sections. Moving from the head to the tailbone, the sections are cervical 104, thoracic 108, lumbar 112, sacral 116, and coccygeal 120. The individual vertebral bodies within the sections are identified by number starting at the vertebral body closest to the head. Of particular interest in this application are the vertebral bodies in the lumbar section and the sacral section. As the various vertebral bodies in the sacral section are usually fused together in adults, it is sufficient and perhaps more descriptive to merely refer to the sacrum rather than the individual sacral components.
It is useful to set forth some of the standard medical vocabulary before getting into a more detailed discussion of the background of the present invention. In the context of the this discussion: anterior refers to in front of the spinal column; (ventral) and posterior refers to behind the column (dorsal); cephalad means towards the patient's head (sometimes “superior”); caudal (sometimes “inferior”) refers to the direction or location that is closer to the feet. As the present application contemplates accessing the various vertebral bodies and intervertebral spaces through a preferred approach that comes in from the sacrum and moves towards the head, proximal and distal are defined in context of this channel of approach. Consequently, proximal is closer to the beginning of the channel and thus towards the feet or the surgeon, distal is further from the beginning of the channel and thus towards the head, or more distant from the surgeon.
The individual motion segments within the spinal columns allow movement within constrained limits and provide protection for the spinal cord. The discs are important to bear and distribute the large forces that pass through the spinal column as a person walks, bends, lifts, or otherwise moves. Unfortunately, for a number of reasons referenced below, for some people, one or more discs in the spinal column will not operate as intended. The reasons for disc problems range from a congenital defect, disease, injury, or degeneration attributable to aging. Often when the discs are not operating properly, the gap between adjacent vertebral bodies is reduced and this causes additional problems including pain.
There are currently over 700,000 surgical procedures performed annually to treat lower back pain in the U.S. In 2004, it is conservatively estimated that there will be more than 200,000 lumbar fusions performed in the U.S., and more than 300,000 worldwide, representing approximately a $1B endeavor in an attempt to alleviate patients' pain. Approximately 60% of spinal surgery takes place in the lumbar spine, and of that portion approximately 80% involves the lower lumbar vertebrae designated as the fourth lumbar vertebra (“L4”), the fifth lumbar vertebra (“L5”), and the first sacral vertebra (“S1”). Persistent low back pain is often attributable to degeneration of the disc between L5 and S1. (See edge between the lumbar region 112 and the sacrum 116 in FIG. 1).
A range of therapies have been developed to alleviate the pain associated with disc problems. One class of solutions is to remove the failed disc and then fuse the two adjacent vertebral bodies together with a permanent but inflexible spacing, also referred to as static stabilization. As mentioned above, an estimated 300,000 fusion operations take place each year. Fusing one section together ends the ability to flex in that motion segment. While the loss of the normal physiologic disc function for a motion segment through fusion of a motion segment may be better than continuing to suffer from the pain, it would be better to alleviate the pain and yet retain all or much of the normal performance of a healthy motion segment.
Another class of therapies attempts to repair the disc so that it resumes operation with the intended intervertebral spacing and mechanical properties. One type of repair is the replacement of the original damaged disc with a prosthetic disc. This type of therapy is called by different names such as dynamic stabilization or spinal motion preservation.
The Operation of the Spine
The bodies of successive lumbar, thoracic and cervical vertebrae articulate with one another and are separated by the intervertebral spinal discs. Each spinal disc comprises a fibrous cartilage shell enclosing a central mass, the “nucleus pulposus” (or “nucleus” herein) that provides for cushioning and dampening of compressive forces to the spinal column. The shell enclosing the nucleus comprises cartilaginous endplates adhered to the opposed cortical bone endplates of the cephalad and caudal vertebral bodies and the “annulus fibrosus” (or “annulus” herein) comprising multiple layers of opposing collagen fibers running circumferentially around the nucleus pulposus and connecting the cartilaginous endplates. The natural, physiological nucleus is comprised of hydrophilic (water attracting) mucopolysacharides and fibrous strands (protein polymers). The nucleus is relatively inelastic, but the annulus can bulge outward slightly to accommodate loads axially applied to the spinal motion segment.
The intervertebral discs are anterior to the spinal canal and located between the opposed end faces or endplates of a cephalad and a caudal vertebral bodies. The inferior articular processes articulate with the superior articular processes of the next succeeding vertebra in the caudal (i.e., toward the feet or inferior) direction. Several ligaments (supraspinous, interspinous, anterior and posterior longitudinal, and the ligamenta flava) hold the vertebrae in position yet permit a limited degree of movement. The assembly of two vertebral bodies, the interposed, intervertebral, spinal disc and the attached ligaments, muscles and facet joints is referred to as a “spinal motion segment”.
The relatively large vertebral bodies located in the anterior portion of the spine and the intervertebral discs provide the majority of the weight bearing support of the vertebral column. Each vertebral body has relatively strong, cortical bone layer comprising the exposed outside surface of the body, including the endplates, and weaker, cancellous bone comprising the center of the vertebral body.
The nucleus pulposus that forms the center portion of the intervertebral disc consists of 80% water that is absorbed by the proteoglycans in a healthy adult spine. With aging, the nucleus becomes less fluid and more viscous and sometimes even dehydrates and contracts (sometimes referred to as “isolated disc resorption”) causing severe pain in many instances. The spinal discs serve as “dampeners” between each vertebral body that minimize the impact of movement on the spinal column, and disc degeneration, marked by a decrease in water content within the nucleus, renders discs ineffective in transferring loads to the annulus layers. In addition, the annulus tends to thicken, desiccate, and become more rigid, lessening its ability to elastically deform under load and making it susceptible to fracturing or fissuring, and one form of degeneration of the disc thus occurs when the annulus fissures or is torn. The fissure may or may not be accompanied by extrusion of nucleus material into and beyond the annulus. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc, and disc fissures can nevertheless be painful and debilitating. Biochemicals contained within the nucleus are enabled to escape through the fissure and irritate nearby structures.
A fissure also may be associated with a herniation or rupture of the annulus causing the nucleus to bulge outward or extrude out through the fissure and impinge upon the spinal column or nerves (a “ruptured” or “slipped” disc). With a contained disc herniation, the nucleus may work its way partly through the annulus but is still contained within the annulus or beneath the posterior longitudinal ligament, and there are no free nucleus fragments in the spinal canal. Nevertheless, even a contained disc herniation is problematic because the outward protrusion can press on the spinal cord or on spinal nerves causing sciatica.
Another disc problem occurs when the disc bulges outward circumferentially in all directions and not just in one location. This occurs when, over time, the disc weakens bulges outward and takes on a “roll” shape. Mechanical stiffness of the joint is reduced and the spinal motion segment may become unstable, shortening the spinal cord segment. As the disc “roll” extends beyond the normal circumference, the disc height may be compromised, and foramina with nerve roots are compressed causing pain. Current treatment methods other than spinal fusion for symptomatic disc rolls and herniated discs include “laminectomy” which involves the surgical exposure of the annulus and surgical excision of the symptomatic portion of the herniated disc followed by a relatively lengthy recuperation period. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and foramina through which nerves pass. The cephalad vertebra may eventually settle on top of the caudal vertebra. This condition is called “lumbar spondylosis”.
Various other surgical treatments that attempt to preserve the intervertebral spinal disc and to simply relieve pain include a “discectomy” or “disc decompression” to remove some or most of the interior nucleus thereby decompressing and decreasing outward pressure on the annulus. In less invasive microsurgical procedures known as “microlumbar discectomy” and “automated percutaneous lumbar discectomy”, the nucleus is removed by suction through a needle laterally extended through the annulus. Although these procedures are less invasive than open surgery, they nevertheless suffer the possibility of injury to the nerve root and dural sac, perineural scar formation, re-herniation of the site of the surgery, and instability due to excess bone removal. In addition, they generally involve the perforation of the annulus.
Although damaged discs and vertebral bodies can be identified with sophisticated diagnostic imaging, existing surgical interventions and clinical outcomes are not consistently satisfactory. Furthermore, patients undergoing such fusion surgery experience significant complications and uncomfortable, prolonged convalescence. Surgical complications include disc space infection; nerve root injury; hematoma formation; instability of adjacent vertebrae, and disruption of muscle, tendons, and ligaments, for example.
Several companies are pursuing the development of prosthesis for the human spine, intended to completely replace a physiological disc, i.e., an artificial disc. In individuals where the degree of degeneration has not progressed to destruction of the annulus, rather than a total artificial disc replacement, a preferred treatment option may be to replace or augment the nucleus pulposus, involving the deployment of a prosthetic disc nucleus. As noted previously, the normal nucleus is contained within the space bounded by the bony vertebrae above and below it and the annulus fibrosus, which circumferentially surrounds it. In this way the nucleus is completely encapsulated and sealed with the only communication to the body being a fluid exchange that takes place through the bone interface with the vertebrae, known as the endplates.
The hydroscopic material comprising the physiological nucleus has an affinity for water (and swells in volume) which is sufficiently powerful to distract (i.e., elevate or “inflate”) the intervertebral disc space, despite the significant physiological loads that are carried across the disc in normal activities. These forces, which range from about 0.4× to about 1.8× body weight, generate local pressure well above normal blood pressure, and the nucleus and inner annulus tissue are, in fact, effectively avascular.
The existence of the nucleus as a cushion (e.g., the nucleus is the “air” in the “tire” known as a spinal disc), and the annulus, as a flexible member, contributes to the range of motion in the normal disc. Range of motion is described in terms of degrees of freedom (i.e., translation and rotation about three orthogonal planes relative to a reference point, the instantaneous center of rotation around the vertical axis of the spine).
Distraction
Pure distraction rarely occurs and is usually a combination of tension and compression on the spinal joints depending on the direction of applied force. An example of a distraction force is therapeutic spinal traction to “unload” the spine.
In the context of the present invention, as used herein the term distraction refers procedurally to an elevation in height that increases the intervertebral disc space resulting from introduction of the motion preservation assembly or prosthetic nucleus device (“PND”), which may be achieved either in the axial deployment of the device itself, or assisted by means of a temporary distraction rod, during implantation. Temporary distraction refers to elevation of disc height by means, such as a distraction rod, which is subsequently removed but wherein the elevation is retained intra-operatively, while the patient remains prone. Thus, the device may be inserted into an elevated disc space first created by other distraction means, and thereafter physical presence and dimensionality of the inserted device is key to preserving that height space, to decompress the disc and alleviate pain caused by nerve impingement.
To date, drawbacks of currently contemplated or deployed prosthetic nucleus devices include subsidence; their tendency to extrude or migrate; to erode the bone; to degrade with time, or to fail to provide sufficient biomechanical load distribution and support. Some of these drawbacks relate to the fact that their deployment typically involves a virtually complete discectomy of the disc achieved by instruments introduced laterally through the patient's body to the disc site and manipulated to cut away or drill lateral holes through the disc and adjoining cortical bone. The endplates of the vertebral bodies, which comprise very hard cortical bone and help to give the vertebral bodies needed strength, are usually weakened or destroyed during the drilling. The vertebral endplates are special cartilage structures that surround the top and bottom of each vertebra and are in direct contact with the disc. They are important to the nutrition of the disc because they allow the passage of nutrients and water into the disc. If these structures are injured, it can lead to deterioration of the disc and altered disc function. Not only do the large laterally drilled hole or holes compromise the integrity of the vertebral bodies, but the spinal cord can be injured if they are drilled too posterior.
Alternatively, current devices are sometimes deployed through a surgically created or enlarged hole in the annulus. The annulus fibrosus consists of tough, thick collagen fibers. The collagen fibers which comprise the annulus fibrosus are arranged in concentric, alternating layers. Intra-layer orientation of these fibers is parallel, however, each alternating (i.e., interlayer) layers' collagen fibers are oriented obliquely (˜120). This oblique orientation allows the annulus to resist forces in both vertical and horizontal directions. Axial compression of a disc results in increased pressure in the disc space. This pressure is transferred to the annulus in the form of loads (stresses) perpendicular to the wall of the annulus. With applied stress, these fibrous layers are put in tension and the angle from horizontal decreases to better resist the load, i.e., the annulus works to resist these perpendicular stresses by transferring the loads around the circumference of the annulus (Hoop Stress). Vertical tension resists bending and distraction (flexion and extension). Horizontal tension resists rotation and sliding (i.e., twisting). While the vertical components of the annulus' layers enable the disc to withstand forward and backward bending well, only half of the horizontal fibers of the annulus are engaged during a rotational movement. In general, the disc is more susceptible to injury during a twisting motion, deriving its primary protection during rotation from the posterior facet joints; however, this risk is even greater if and when the annulus is compromised.
Moreover, annulus disruption will remain post-operatively, and present a pathway for device extrusion and migration in addition to compromising the physiological biomechanics of the disc structure. Other devices, in an attempt to provide sufficient mechanical integrity to withstand the stresses to which they will be subjected, are configured to be so firm, stiff, and inflexible that they tend to erode the bone or become imbedded, over time, in the vertebral bodies, a phenomenon known as “subsidence”, sometimes also termed “telescoping”. The result of subsidence is that the effective length of the vertebral column is shortened, which can subsequently cause damage to the nerve root and nerves that pass between the two adjacent vertebrae.